This application claims the benefit of Korean Patent Application No. 1999-051594, filed on Nov. 19, 1999, the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein, and the benefit of Korean Patent Application No. 1999-0052213, filed on Nov. 23, 1999, the entirety of which is also hereby incorporated by reference for all purposes as if fully set forth herein, both under 35 U.S.C. xc2xa7 119.
1. Field of the Invention
The present invention relates to a liquid crystal cell design for a liquid crystal display (LCD) device, and more particularly, to a liquid crystal cell that uses a twisted nematic (TN) liquid crystal.
2. Discussion of the Related Art
Conventional LCD devices include display panels. Such display panels have upper and lower substrates that are attached with each other, and a liquid crystal, such as a twisted nematic (TN) liquid crystal, interposed there between. Such display panels are operationally divided into a plurality of liquid crystal cells. On exterior surfaces of the upper and the lower substrates, polarizers and retardation films, or compensation films are selectively attached.
A major consideration in the design of liquid crystal cells is the characteristics of the particular liquid crystal that is used. A good liquid crystal should have a short response time, a low gray scale, a wide viewing angle, and should be operational at low voltages. However, it is very difficult to find a liquid crystal that has all of these characteristics. Thus, various designs have been adopted for liquid crystal display devices.
Among the various types of TN liquid crystals, a low twisted nematic (LTN) liquid crystal has advantages of a short response time and a good gray scale. However, it typically has a low contrast ratio and relatively poor color dispersion properties. Other twisted nematic (TN) liquid crystals have twisted angles of 90 degrees, or those employing an in-plating switching (IPS) mode. Those liquid crystals can provide a wide viewing angle, but afterimages are produced during moving images and their brightness is relatively low. The anti-ferroelectric liquid crystal (AFLC) or an optical compensated birefringence (OCB) have advantages of a wide viewing angle and a short response time, although there are problems with contrast ratios and cell gap alignment.
Of particular interest to this invention is the difficulty of determining the optimum design parameters of a liquid crystal cell. A liquid crystal cell design should take into consideration many parameters, including liquid crystal arrangement and the transmittance axis directions of the polarizers. However, as there are simply too many important factors to consider it is humanly impossible to consider them all. Accordingly, computer simulation is usually used to process the design parameters and to arrive at an optimum liquid crystal cell.
One such computer simulation is the parameter space approach. The parameter space approach provides a graph that illustrates transmittance with respect to the product of a cell""s thickness and birefringence when under a non-electric field condition. In the parameter space graph, the optimum parameter values of cell thickness and birefringence product xe2x80x9cdxcex94nxe2x80x9d, or xe2x80x9cd.DELTA.nxe2x80x9d where the transmittance is highest can be easily found. The d.DELTA.n is calculated using a Jones matrix formulation.
As the Jones matrix formulation (and the generalized geometric optics approximation [GGOA]) has been fully discussed elsewhere it need not be repeated in detail here. An important point to note is that in the Jones matrix formulation the liquid crystal director (the direction in which the molecules line up) is assumed to be uniform over the entire cell. However, it is well known that the tilt angle decreases in the middle of the liquid crystal cell due to elastic energy minimization, especially for high pretilt angle cases. However, since an average tilt angle can be used without producing any significant error in predicting the properties of the LCD, most computer simulations assume that the tilt angle is zero.
The basic configuration and operation of a twisted nematic liquid crystal device will be provided. Then, a more detailed description of the parameter space method will be given. As shown in FIG. 1, first and second polarizers 10 and 16, respectively, having first and second transmittance axis directions 40 and 42 that are perpendicular to each other, are opposed with and spaced apart from each other. Between the two polarizers 10 and 16 are first and second transparent substrates 12 and 14, which are also opposed with and spaced apart from each other. Spacers are used to maintain the cell gap between the substrates. For example, plastic balls or silica balls having a diameter of 4 to 5 micrometers can be sprayed on the first substrate.
Still referring to FIG. 1, the first and the second transparent substrates 12 and 14 include first and second orientation films 20 and 22, respectively, on their opposing surfaces. Between the first and the second orientation films 20 and 22 is a positive TN liquid crystal 18.
The positive TN liquid crystal has a characteristic that it becomes arranged according to the direction of an applied electric field. The first and the second polarizer 10 and 16, respectively, transmit light that is parallel with their transmittance-axis directions 40 and 42, but reflect or absorb light that is perpendicular to their transmittance-axis directions 40 and 42.
The first and the second orientation films 20 and 22 were previously rubbed in a proper direction with a fabric. This rubbing causes the positive TN liquid crystal molecules between the first and the second transparent substrates 12 and 14 to become tilted several degrees from each substrate surface. First and second rubbing directions 50 and 52 of the first and the second orientation films 20 and 22 are, respectively, parallel with the transmittance-axis directions of the first and the second polarizer 10 and 16. When no electric field is applied to the positive TN liquid crystal 18, the orientation of the liquid crystal molecules becomes twisted from one substrate to the other at a definite angle, that being the twisted angle of the positive TN liquid crystal 18.
During operation, a back light device 24 irradiates white light onto the first polarizer 10. The first polarizer 10 transmits only the portion of the light that is parallel with the first transmittance-axis direction 40. The result is a first linearly polarized light 26 that passes through the polarizer 10. The first linearly polarized light 26 then passes through the positive TN liquid crystal 18 via the first transparent substrate 12.
As the first polarized light 26 passes through the positive TN liquid crystal 18, the first linearly polarized light 26 changes its phase according to the twisted alignment of the positive TN liquid crystal molecules. Accordingly, the first linearly polarized light 26 becomes an elliptically (possibly circularly) polarized light 28.
The elliptically polarized light 28 passes through the second transparent substrate 14, and meets the second polarizer 16. When the elliptically polarized light 28 passes through the second polarizer 16, the second polarizer 16 transmits only the portion of the elliptically polarized light 28 that is parallel to the second transmittance-axis direction 42. A polarized light 30 is then emitted. At the above-mentioned operation mode, a white state is displayed.
Turning now to FIG. 2, when a voltage supplier 35 induces an electric field through the positive TN liquid crystal 18, the positive TN liquid crystal molecules rotate and become arranged such that the longitudinal axes of the molecules become perpendicular to the surfaces of the first and second substrates 12 and 14. Accordingly, the first linearly polarized light 26 passes through the first transparent substrates 12, the positive TN liquid crystal 18, and the second transparent substrate 14 without phase change. The first linearly polarized light 26 then meets the second polarizer 16. As the second polarizer 16 has the second transmittance-axis direction 52 that is perpendicular to the first linearly polarized light 26, the second polarizer 16 absorbs or shields most of the first linearly polarized light 26. Thus, little or none of the first linearly polarized light 26 passes through the second polarizer 16. Accordingly, a dark state is displayed.
The conventional parameter space approach will be explained in some detail with references to FIGS. 3 and 4. FIGS. 3 and 4 show transmittance graphs of a liquid crystal cell according to FIGS. 1 and 2. In the transmittance graphs, white and black regions illustrate the highest and the lowest transmittances of the liquid crystal cell, respectively.
For FIG. 3, the rubbing directions of the first and the second orientation films are, respectively, parallel with the transmittance axis directions of the first and the second polarizers. At a portion xe2x80x9cAxe2x80x9d the corresponding twist angles are below 90 degrees, additionally, no white region appears, which means that there is no acceptable optimum d.DELTA.n.
For FIG. 4, the angles between the rubbing directions of the first and the second orientation films and the transmittance axis directions of the first and the second polarizers are, respectively, xe2x88x9245 and 45 degrees (or vice versa). A portion xe2x80x9cBxe2x80x9d, where the corresponding twist angle is about 45 degrees, is dark. This also means that there is no acceptable optimum d.DELTA.n.
Accordingly, if the twist angle of the TN liquid crystal is below 90 degrees, the optimum d.DELTA.n can not be found in the transmittance graph produced from the conventional parameter space approach.
Accordingly, the principles of the present invention relate to liquid crystal cells that are designed to substantially obviate one or more of the problems due to the limitations and disadvantages of the related art.
It is an object of the present invention to provide a liquid crystal display device that has a short response time, a wide viewing angle, and a low gray scale.
It is another object of the present invention to provide a fabricating method for the same liquid crystal display device.
In view of the foregoing and other problems of the conventional design methods, it is an object of the present invention to provide a liquid crystal cell that includes first and second substrates that are spaced apart from and opposed to each other. The liquid crystal cell further includes first and second orientation films that have, respectively, first and second rubbing directions and that are positioned, respectively, on opposing surfaces of the first and the second substrates. First and second polarizers that have, respectively, perpendicular first and second transmittance axis directions are positioned, respectively, on the outer surfaces of the first and the second substrates. A liquid crystal having a twisted angle xe2x80x9cxc3x8xe2x80x9d (assumed to be continuous in the liquid crystal cell) is interposed between the first and the second orientation films, with the first and the second orientation films being separated by a gap xe2x80x9cdxe2x80x9d. The first and the second rubbing directions form an angle xe2x80x9cxc3x8xe2x80x9d. Furthermore, the first rubbing direction is at an angle of (90xe2x88x92xc3x8)/2 with the first transmittance axis direction, and the second rubbing direction is at an angle of (90xe2x88x92xc3x8)/2 with the second transmittance axis direction.
The gap xe2x80x9cdxe2x80x9d is set (by design) to provide optimum transmittance with the angle (90xc3x8)/2 as a factor. Optimum transmittance, and thus the gap xe2x80x9cd,xe2x80x9d is determined using a Jones matrix and a parameter space approach.
In another aspect, the principles of the present invention provide a liquid crystal display device including first and second substrates that are spaced apart from and opposed with each other, and first and second orientation films, respectively, positioned on opposing surfaces of the first and second substrates. The first and second orientation films having first and second rubbing directions, respectively. A TN liquid crystal having a twisted angle xe2x80x9cxc3x8xe2x80x9d is interposed between the first and second substrates and a compensation film is positioned on an outer surface of the second substrate. A first polarizer having a first transmittance axis direction is located on an outer surface of the first substrate such that the first transmittance axis direction makes an angle of (90xe2x88x92xc3x8)/2 degrees with the first rubbing direction of the first orientation film. A second polarizer having a second transmittance axis direction is located on an outer surface of the compensation film such that the second transmittance axis direction is perpendicular to the first transmittance axis direction and makes an angle of (90xe2x88x92xc3x8)/2 degrees.
The liquid crystal display device further includes patterned spacers between the first and second substrates, wherein the spacers have a height of 2 micrometers.
The compensation film beneficially has a phase difference of 10 to 60 nanometers.
In another aspect, the present invention provides a method for fabricating a liquid crystal display device. The fabricating method includes preparing first and second substrates, forming first and second orientation films, respectively, on a surface of the first and second substrates, and rubbing the first and second orientation films to create first and second rubbing directions. Spacers are patterned on the first orientation film. The fabricating method further includes attaching the first and second substrates together such that the first and second orientation films oppose each other, inserting a TN liquid crystal having a twisted angle xe2x80x9cxc3x8xe2x80x9d between the first and second orientation films, and attaching a compensation film to an outer surface of the second substrate. Then, first and second polarizers are respectively attached to the outer surfaces of the first substrate and to the compensation film. The first and second polarizers, respectively, having first and second transmittance axis directions, with the first and second transmittance axis directions being perpendicular to each other and making an angle of (90xe2x88x92xc3x8)/2 with the first and second rubbing direction, respectively. Beneficially, the spacers have a height of 2 micrometers.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. However, it should be understood that the written description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.